Memory devices are typically provided as internal, semiconductor, integrated circuits in computers or other electronic devices. There are many different types of memory including random-access memory (RAM), read only memory (ROM), dynamic random access memory (DRAM), synchronous dynamic random access memory (SDRAM), and flash memory.
Flash memory integrated circuits typically require relatively large voltages for programming and erasing operations. For example, the memory IC may have a supply voltage of 3V but require a program voltage of 20V.
FIG. 1 illustrates a typical prior art high voltage switching circuit. This circuit is composed of an enhancement mode n-channel metal oxide semiconductor field effect transistor (MOSFET) 101 connected in series to an enhancement mode p-channel MOSFET 102. An n-channel depletion mode MOSFET 103 is connected between the enhancement transistors 101, 102 and the high voltage, VPP, to be switched. The gate of the depletion transistor 103 is connected to VOUT. The substrate or well of the enhancement PMOSFET 102 is connected 105 to the source of the depletion NMOSFET 103. An inverter 100 inverts the VIN signal.
A logical one signal for VIN is inverted by the inverter 100 to a logical zero. This turns off the enhancement mode NMOSFET 101 and VOUT is charged to VPP through the enhancement mode PMOSFET 102 and the depletion mode NMOSFET 103. The substrate voltage 105 of the PMOSFET 102 is also at VPP.
When VIN is a logical zero, the inverter 100 inverts the signal to a logical one that is applied to the enhancement mode NMOSFET 101. This turns on the NMOSFET 101 thus causing the circuit to discharge to circuit ground, VSS. This causes the gate potential of the depletion NMOSFET 103 to be 0V, turning off that transistor 103. The substrate/well voltage of the enhancement PMOSFET 102 is thus 0V. The gate bias for this transistor 102 is 5V (i.e., logical 1) but since the potential of the substrate is smaller than the 5V of the input signal, the PMOSFET 102 will cut off.
FIG. 2 shows a typical example of the relationship between input and output signals of the circuit of FIG. 1. It can be seen that VIN at the bottom goes to VCC causing the VOUT signal to go to VPP.
One problem with the prior art switching circuit is that the PMOSFET 102 experiences a large gate to substrate voltage 105. After an extended period of time under this bias, the electron or hole injection causes the threshold voltage, Vth, to vary as illustrated in FIG. 3. This can cause the switching circuit to fail to turn on if the Vth decreases or increase the leakage current of the circuit if Vth increases.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for a switching circuit having an improved reliability.